The SAS has some P226's for anti-terrosit operations. Most other operations don't require a sidearm, and if they do they carry the standard UK sidearm of the BHP. I hadn't heard about the Beretta, but they may very well have bought some.
Remember that the UK has a much smaller budget than the US, so there's not as much willingness to go out and buy loads of new equipment for a single Regiment over other operational budget concerns. They buy what they need when they have a good reason, and they make do with what they already have when they can. Only the US has the kind of money to burn like we do.
20rd mags are available for both the P226 and the BHP. I have both factory and Mecgar 20rd mags for my P226 and they are still around, but expensive now. The BHP 20 rounders are less expensive, and work great as well. They stick out about an inch or so from the bottom on both guns.
On of my BHP's has an extended safety I fitted to it. It's from Cylinder and Slide. There is about any item available for the BHP that's available for any other handgun. The C&S safety is high quality and was easy to fit. I found the difference to be night and day compared to the old safety. It's easy to use, and it "clicks" into place, just like a Govt Model. I bought mine off of Auction Arms. I know Brownells and all the other list of usual suspects carries them as well.
Ross